Crystal controlled oscillator circuits



06b2, 1934. O PETERSON 1,975,615

CRYSTAL CONTROIJLED OSCILLATOR CIRCUITS Original Filed March 2, 1932 $0OUTPUT INVENTOR new PETERSON I BY ATTORNEY Patented Oct. 2, 1934 UNITEDSTATES CRYSTAL CONTROLLED OSCILLATOR c accrrs Harold Olaf Peterson,Riverhead, N. Y., assignor to Radio Corporation of America, acorporation 1 of Delaware Application March 2,

1932, Serial No. 596,197

Renewed February 24, 1934 5 Claims.

In Figure 3 of Cady United States Patent 1,472,583, there is describedan efficient crystal controlled oscillator. However, it will be foundthat there is some tendency for this circuit to oscillate at frequencieswhich do not correspond to the desired crystal frequency, or in otherwords, there is some tendency to oscillate at spurious frequencies andat these frequencies the crystal acts merely as a dielectric ratherthan, as desired, a frequency controlling element.

To render the system absolutely fool-proof and cause it to oscillateonly at a frequency corresponding to that of the crystal frequency isthe principal object of my present invention. To eliminate the tendencyto oscillate at spurious frequencies, I place or connect, according tomy present invention, an impedance of relatively low value directlyacross the grid filament circuit of the crystal control system such thatspurious frequency voltages cannot be built up across the grid andfilament. However, at the frequency corresponding to that of thecrystal, due to the series resonance effect of the crystal incombination with the action of the parallel tuned circuit also in thegrid circuit, control potentials at the natural frequency of the crystalare set up between the grid and filament or between the controlelectrode and cathode to cause the generation of oscillations of thedesired or crystal frequency. I

My invention is more fully described in connection with the accompanyingdrawing wherein;

Figure 1 illustrates a preferred form of my present invention; and iFigure 2 illustrates a modified form utilizing a screen grid tube.

Turning to Figure 1, I have indicated an electron discharge device orvacuum tube 2 having a cathode 4 energized through transformer '7 withalternating heating currents. By means of the condenser network 6 andground connection 8, symmetry of the heating circuit is insured,eliminating to a desired degree possible modulation which wouldotherwise occur due to the alternating E. M. F. applied to the filamentheating circuit by means of transformer '7. Between the cathode 4 andgrid or control electrode 10 there is connected a piezo-electricfrequency controlling crystal 12 as well as a parallel tuned circuit 14.As in the Cady patent referred to, some feed back occurs through theinter-element capacity of the tube 2 between plate or anode 15 thereofand the grid 10 as well as through the feed back or tickler coil 18. Itis preferable that shielding means 20 be provided between the coil 18and the inductance coil 22 of tunable circuit 14 as well as providingfor the grounding of the direct current end of the tickler coil 18 bymeans of condenser 24. Grid has is provided by the grid leak resistor 26effectively in shunt with the capacity formed by the electrodes ofpiezo-electric frequency controlling crystal 12.

As so far described, the system, in general, will act as an excellentcrystal controlled oscillator. However, at certain frequencies, therewill be a tendency to generate spurious oscillations deter mined by theelectrical constants of the circuit, the crystal being considered simplyas a di-electric rather than a piezo-electric crystal. To eliminate thetendency to oscillate at these spurious frequencies I connect in shuntto, or, across the grid and cathode circuit at points A and B a lowimpedance in the form of a condenser 28 or, as shown in Fig. 2, ainductor 34 in series with a large blocking condenser 36.

Now, referring back to Figure 1 again, should there be any tendency forthe circuit to oscillate at some parasitic or spurious frequency, thoseundesired frequencies will be by-passed by the low impedance between thepoints A and B, and no energy of spurious frequencies will occur in theoutput circuit so designated on the drawing. However, at the naturalfrequency of the crystal, the crystal acts as a series tuned circuit,thereby presenting a path of lower impedance than that between thepoints A and B and thereby causing currents to flow in the paralleltuned circuit 14. These currents,of course, build up electron streamcontrol voltages of desired frequency across the grid and filament andthereby cause energy of desired frequency to appear in the outputcircuit.

The purpose of the condenser in the series combination connected acrosspoints A and B of Figure 2 is to prevent short circuiting of the gridleak resistor 26. The grid leak resistor may be replaced by the seriescombination of a source of potential and an. impedance to prevent theflow of alternating current therein, the impedance taking the form of,for example, a choke coil or resistor.

Figure 2 illustrates a modification wherein the tickler or feed-backcoil or inductor 18 is tuned by means of a variable condenser 40 andwherein the crystal 12 is placed on the -otherwise grounded side of theparallel tuned circuit 14. To prevent inter-electrode feed-back, tube 2is chosen of the screen grid type having a screen grid 42. For betterfrequency stability and as an added refinement, the cathode 4 may beenergized from a direct current potential circuit diagrammati- 1 callyindicated at 44. Condensers 25 and 52 of Fig. 2 are by-pass radiofrequency condensers. Preferably in Fig. 2, shielding means 20 is usedto prevent, to any desired extent, capacitive feed back between thetickler coil 18 and the coil 22 of the parallel tuned circuit 14. befound in the circuit shown in Fig. 2, that by suitable adjustment of thetuning of circuit 14, and with a capacity across points A and 3 feedback through the tickler coil 18 may be reversed so as to be ordinarilydegenerative and the circuit will operate highly satisfactorily undercrystal control. This may be explained by virtue of the fact thatdespite the degenerative feed-back, suitable tuning of tuned circuit 14Will alter the phase of the voltages fed to the control grid 10 of tube2 of Fig. 2, such that they are in correct phase for the production ofoscillations at the frequency corresponding to that of crystal 12.

Also in Fig. 2, the impedance formed by inductor 34 and large by-passcondenser 36 is low for undesired frequencies. However, for energy of afrequency corresponding to that of the crystal 12, the crystal 12 actsas a circuit of even lower impedance and draws current through theparallel tuned circuit 14 at the desired frequency. Consequently,sufficient voltage of the desired frequency to control the electronstream within the tube is built up on the grid 10 of tube 2 so as toproduce in the output leads oscillations of a frequency corresponding tothat of only the crystal.

The condenser 52, provided in the output circuit, is a blockingcondenser which allows passage of the high frequency energy generatedbut prevents passage of the direct current voltage applied to the plateor anode 15.

It is to be noted that the inductive feed back shown in Fig. 1 may bereversed, and yet have oscillation generation at the frequency of thecrystal. Such action is had by virtue of suitable tuning of paralleltuned circuit 14. In that case it will be found that the reversedfeed-back will entirely overcome the regenerative feed-back through theinter-element capacity of the tube 2 and yet, because of the phasecontrol of circuit 14 the system will be found to oscillate at thedesired frequency only.

In Fig. 2, as already indicated, the crystal 12 has. been placed betweenground and one terminal of parallel tuned circuit 14. This arrangementoffers some advantages in the mechanical construction of the oscillatingsystem described in that careful insulation of all of the crystalelectrodes from ground is not required.

However, it will Having thus described my invention, what I claim is:

1. A vacuum tube oscillation generator comprising a vacuum tube having afilament, a grid, and a plate, a piezo-electric crystal in series with aparallel tunable circuit connected between said grid and said filament,and, a relatively low impedance connected across said grid and filamentwhereby only oscillations of a frequency corresponding to the naturalfrequency of said piezoelectric crystal are generated by said electrondischarge device.

2. A crystal controlled oscillation generator comprising an electrondischarge device having an anode, a cathode and a control electrode, the

series combination of an electro-mechanical vibrator of relativelyconstant frequency and a high resonant impedance having inductance andcapacity connected between said control electrode and cathode, arelatively low impedance connected across said grid and cathode, andmeans for establishing feed back between the anode and control electrodeof said device whereby oscillations corresponding to the frequency ofsaid electro-mechanical vibrator are generated by said device.

3. In apparatus of the character described, a vacuum tube having ananode, a cathode and a control electrode, a parallel tuned circuit and apiezo-electric crystal connected in series, the said series combinationbeing connected between said control electrode and cathode, a, grid leakresistor connected across said control electrode and cathode, arelatively low impedance connected across said cathode and controlelectrode, and, means for establishing feed-back between the controlelectrode cathode circuit and said anode cathode circuit of saidelectron discharge device whereby oscillations of a frequencycorresponding to that of said piezo-electric crystal are generated bysaid electron discharge device.

4. Apparatus as defined in claim 3 wherein said low impedance element isin the form of a condenser.

5. Apparatus as claimed in claim -3 wherein means are provided toprevent inter-electrode HAROLD OLAF PETERSON.

